Various joint prosthesis components include elongate stems that are to be mounted within the intramedullary canal of a bone while the other end is attached to another prosthesis component that is mounted upon the bone. Such stems are used, for example, with femoral knee stems and tibial knee stems.
Knee arthroplasty procedures involve the installation of a femoral component on the patient's femur and a tibial component on the patient's tibia. The tibial component usually comprises a tibial stem which is attachable to a tibial tray. The tibial stem is designed to be installed within the intramedullary canal of the tibia while the tibial tray mounts upon a prepared surface on the head of the tibia. A tibial bearing member, which articulates with the femoral component, is typically mounted upon the tibial tray.
The variations in the human anatomy of different patients, especially in bones such as the tibia, creates the need for a variety of implant sizes and configurations. In some instances, it is necessary that the longitudinal axis of a stem component, such as a tibial stem, be laterally offset from the longitudinal axis of the other prosthesis component, such as the tibial tray. In many individuals however, these axes must be offset with respect to one another to ensure proper implantation. Even where offset is required, there is no uniformity as to the degree or direction of offset.
Modular prosthesis systems have been developed to accommodate the variability in patient anatomies. Modular systems include a number of interchangeable parts, each having different sizes or other physical characteristics. Such modular systems are useful in that they allow surgeons to use one or more standard parts with interchangeable components having different characteristics.
U.S. Pat. No. 5,290,313 discloses a modular tibial prothesis in which a tibial stem is mounted so as to be laterally offset with respect to the longitudinal axis of a tibial tray. A coupling allows specially designed tibial stems to be mounted to the tibial tray to achieve a desired offset orientation. One disadvantage of this design is that the tibial stems themselves are offset, and a different stem must be used to achieve a desired offset orientation. As a result, a number of different, non-standard tibial stems are needed to achieve the desired offset orientation required for a given patient. Such a system can increase the cost of prostheses because several non-standard parts are necessary to cope with all possible anatomical requirements of patients.
Thus it would be desirable to have a modular prosthesis system which could accommodate the variability in patient anatomies and allow greater flexibility to the surgeon, while still providing the ability for the tibial bearing insert to rotate with respect to the tibial tray.